Air-conditioning apparatuses have been available in which a plurality of indoor units are connected to one or more outdoor units and which is capable of performing a cooling operation in which each indoor unit performs cooling only, a heating operation in which each indoor unit performs heating only, and a mixed operation in which indoor units perform cooling and heating simultaneously. In such an air-conditioning apparatus, for heating when the outside air has a low temperature, in general, the suction density of a compressor decreases, and the flow rate of refrigerant and the heating capacity decrease. Thus, the heating capacity may be increased by injecting a refrigerant in the course of a compression process by the compressor. For example, Patent Literature 1 describes a refrigerant circuit of a cooling and heating simultaneous air-conditioning apparatus including a compressor which allows injection.
Furthermore, in recent years, in view of global environmental protection, as a refrigerant to be used for air conditioning, switching from a currently-used refrigerant having a high global warming potential (GWP), such as an R410 refrigerant, an R407C refrigerant, or an R134a refrigerant, to a refrigerant having a low GWP, such as a carbon dioxide refrigerant, an ammonia refrigerant, a hydrocarbon-based refrigerant, a tetrafluoropropane (HFO)-based refrigerant, or a difluoromethane (R32) refrigerant, is in progress. Among the refrigerants having low GWP, an R32 refrigerant has substantially the same evaporating and condensing pressures as an R410A refrigerant and has a refrigeration capacity per unit mass and volume greater than the R410A refrigerant. Therefore, with the R32 refrigerant, the apparatus can be downsized.
Thus, adoption of an R32 refrigerant or a mixture of an R32 refrigerant and an HFO refrigerant or the like has been regarded as being promising. However, the R32 refrigerant has a property of making a compressor have a smaller suction density and a higher discharge temperature than the R410A refrigerant. For example, in the case where the evaporating temperature is 5 degrees Centigrade, the condensing temperature is 45 degrees Centigrade, and the degree of superheat of refrigerant at the time of suction of the compressor is 1 degree Centigrade, the discharge temperature of the R32 refrigerant rises by about 20 degrees Centigrade compared to the R410A refrigerant. The upper limit of the discharge temperature of a compressor is determined based on the guarantee temperature of refrigerating machine oil, sealing material, or the like. In the case where switching to an R32 refrigerant or a refrigerant mixture containing an R32 refrigerant as a component is made, it is necessary to take measures to lower the discharge temperature. Thus, lowering the discharge temperature by injection is effective.